Alkaline membrane fuel cells (AMFCs) have important advantages over other low temperature fuel cells, including the ability to operate with non-precious metal catalysts and without added liquid electrolyte. However, an important challenge to the implementation of this fuel cell technology is the performance loss incurred when CO2 enters the cell. When the AMFC operates on hydrogen fuel, the CO2 in the cathode air feed is a specific source of concern, as the air feed contains around 400 ppm CO2. This “air CO2” will enter the cell continuously through the cell cathode, as long as such untreated air supplies the cathode. Under such conditions of continuous inflow of CO2 at a partial pressure of about 10−4 Pair into the cell cathode, and from the cathode into the cell, significant AMFC voltage losses have been recorded. The cell voltage at constant current density of about 0.2 A/cm2-0.4 A/cm2 is found to be lower by 0.1V-0.3V (in contrast to the same cell operating with a CO2-free cathode air feed), and has been shown to amount to a lowering of the energy conversion efficiency by 20%-60%.
One reason for this fall in AMFC performance is understood to be an acid-base process. CO2 entering the cell recombines with the basic function of the polymer electrolyte to replace the OH− ion-conducting function with a HCO3− (bicarbonate) ion conducting function according to:CO2+(R4N+OH−)=(R4N+HCO3−)  (1)
R4N+ is a tetra-alkyl ammonium ion, the typical immobilized cationic group in an alkaline ionomer. After entering the cell cathode in gaseous form, CO2 can migrate through the thickness dimension of the cell in water-dissolved form, and can propagate the “carbonation process” shown by equation (1) throughout the membrane and the anode of the cell.
Another mode of propagation of the carbonation process through the thickness dimension of the cell is an anion-replacement process. In this case, a bicarbonate anion migrates through the ionomer under current, displacing an OH− anion according to:HCO3−+(R4N+OH−)=(R4N+HCO3−)+OH−  (2)
This occurs while OH− ions in the AMFC migrate towards the cell anode and the anode process consumes OH− ions according to:H2+2OH−=2H2O+2e  (3)where the HCO3− anion is not reactive at the anode under ordinary AMFC operation conditions.
Consequently, the ion-replacement process (2), occurring while the anode consumers OH− ions, will end up in lasting carbonation of a large fraction of the anionic sites.
Replacement of the OH− anion by HCO3− may cause significant AMFC losses for two reasons. First, the mobility of the bicarbonate ion is about 4 times smaller than that of the OH− ion, causing a drop of conductivity in both the cell membrane and the inner-electrode ionomer components. A second reason is the carbonation of OH− ions within the anode. With the OH− ion serving as a reactant in the anode process, lowering its availability for the anode process, as shown in equation (3), results in a significant increase of the anode over-potential.
Electrolyte carbonation is well documented as a significant challenge in alkaline fuel cells (AFCs) based on liquid alkaline electrolytes, e.g., aqueous KOH. The nature of the problem and the solutions required, however, are different in AFCs and in AMFCs. In the case of the AFC, the ultimate result of electrolyte carbonation is the formation of solid carbonate in the liquid electrolyte that needs to be removed continuously. This is typically accomplished with continuous electrolyte recirculation and solid/liquid separation. In the AMFC, no solid carbonate can be formed, which eliminates the need for liquid recirculation and solid carbonate removal. However, the reaction of air CO2 with the liquid alkaline electrolyte to form solid carbonate provides a CO2 sequestration function within the cell. Because the AMFC does not have such in-the-cell CO2 sequestration function, the ionomer material in the AMFC becomes highly vulnerable to air CO2 and the carbonation processes shown in equations (1) and (2) readily convert the ionomer on entry of untreated air from an OH− ion form to a carbonate ion form. Therefore, blocking entry of CO2 and use of remediation tools with an alkaline fuel cell that suffers some degree of carbonation must consequently be effective in securing the cell's immunity to air CO2.
Other than electrolyte recirculation, the traditional approach to minimize the effects of CO2 in alkaline fuel cells has been the upstream use of air scrubbers containing aqueous alkaline solutions or solid CO2 absorbers consisting of granules of alkali and/or alkaline earth hydroxides, such as disclosed in U.S. Pat. No. 3,909,206. When passing through such scrubber or absorber filters, the CO2 component in the air feed stream reacts with the OH− ions in such CO2 trap to form carbonates and thereby to reduce the concentration of CO2 in the air entering the cell. This mode of CO2 filtration occurs upstream from the cell cathode and requires periodic replacement of the filter or of the active material in the filter. The frequency of such manual replacements cannot be too great in most fuel cell applications because of the need to minimize fuel cell maintenance. One possible way to lower the frequency of filter replacements is to use filters having a larger volume, i.e., larger CO2 absorption capacity. However, the permissible size of the filter will be limited by the overall system volume constraints.
Thus, an effective CO2 filter or trap having a combination of a limited, but high capacity, volume and a capacity to maximize a reduction of CO2 levels in an air stream by passing the air stream through such a filter or trap is desirable to minimize CO2 levels in the air stream and within an alkaline fuel cell.